The shrinking of metal oxide semiconductor field effect transistor (MOSFET) dimensions for high density, low power and enhanced performance requires reduced power supply voltages. As a result, dielectric thickness and channel length of the transistors are scaled with power supply voltage.
A static random access memory (SRAM) is a significant memory device due to its high speed, low power consumption, and simple operation. Unlike a dynamic random access memory (DRAM) cell, the SRAM does not need to regularly refresh the stored data and it has a straightforward design. However, SRAM stability is severely impacted by scaling. Small mismatches in the devices during processing can cause the cell to favor one of the states, either a ‘1’ or a ‘0’. Mismatches can result from dislocations between the drain and the source or from dopant implantation or thermal anneal temperature fluctuation.
The SRAM cell stability determines the soft-error and the sensitivity of the memory cell to variations in process and operating conditions. One important parameter for the stability is called “beta ratio”, which is the ratio between pull-down transistor drive current and pass-gate transistor drive current. In other terms, “beta ratio” can be defined as the ion ratio between the pass-gate transistor and pull-down transistor of a SRAM cell. A high beta ratio on the order of about 1.0 or greater is helpful in improving the stability of the SRAM cell.
Conventional methods to adjust the beta ratio include threshold voltage Vt implantation and active area sizing. The latter of which is limited by groundrules and mask sets.
Stress engineering has been used to improve device performance of FET devices. Usually, tensile strain is employed for n-type FETs and compressive stress is employed for p-type FETs to improve device performance. Such stress engineering is typically applied to both the pull-down transistors and the pass-gate transistors. In such prior art structures, improved device performance of both the pull-down and pass-gate transistors can be obtained, however, the beta ratio is not improved.
In view of the above, there is a need for obtaining SRAM cells wherein the beta ratio between the pull-down and pass-gate transistors is significantly improved thereby improving the overall stability of the structure.